Chesapeake Times, Vol 10 | July 2022
Recently, a client reported that their sidescan data did not properly overlap when projecting the sonar imagery using either the sensor or course made good (CMG) setting—the two heading options available in SonarWiz. In the end, it turned out that the sidescan compass had not been adjusted for magnetic declination. Once we applied the appropriate correction in SonarWiz, the data snapped into alignment and the client was happy. I realized while working though this ticket that some people do not understand how the heading projection options in SonarWiz work. I was also surprised that the way I was taught to compute the declination of a survey area is going extinct.
Projection Settings In SonarWiz
SonarWiz projects sidescan beams perpendicular to fish heading. There are two options for determining the heading of the fish: (1) Course made good (CMG) – this assumes the fish heading follows the course of the vessel through the water, and (2) Sensor heading – this uses the fish heading recorded in the ping records
As shown in Figure 1, when projecting the beams using CMG, the beams will be plotted perpendicular to the vessel path through the water. The principal advantage of this method is that it does not require a heading sensor in the fish, the disadvantage is that it cannot account for the actual fish orientation and it usually requires smoothing of the navigation to prevent unsightly jumps in the beams. The second, and better option is to use the sensor heading in the fish. When properly calibrated, the sensor heading measures the true orientation of the fish through the water independent of the fish position or CMG and should provide more accurate image positioning in SonarWiz. However, most embedded heading sensors use a magnetic compass which must be corrected for the angular difference between magnetic north and true north—the magnetic declination. Failure to apply this correction can lead to a fish heading error of up to 20-degrees in some parts of the world. So, where do you look up the declination angle of your survey area and how do you apply the correction in SonarWiz?
Determining Magnetic Declination
Traditional navigation relied on the magnetic compass for heading information and nautical charts always contained a compass rose which indicated the difference between true north and magnetic north. This is where I was trained to look for the magnetic declination of my survey areas. However, modern navigation (GNSS) no longer relies on magnetic compasses and chart plotters have replaced paper charts. In fact, NOAA will no longer publish paper charts after 2025. So, where do we go to find the magnetic declination of our survey areas once paper charts are extinct?
The best resource I have found is the Magnetic Field Calculator from the NCEI (NOAA). The web site is here:
https://ngdc.noaa.gov/geomag/calculators/magcalc.shtml?useFullSite=true
This appears to be a world-wide resource. As shown in Figure 3, the magnetic declination of the same area shown in Figure 2, is now 15.62 degrees east of north. A change of -1.63 degrees since the paper chart was published in 2017.
At 100 m range, forgetting to apply the magnetic declination offset will mis-position your targets by more than 27 m. This was the root problem our client experienced at the beginning of this article.
Keep in mind that the magnetic pole is moving quickly over time. You need to use updated declination values. For example, if we used the 2017 declination value published on the paper chart in 2017 without adjusting for annual variation, our targets would be mis-positioned by 2.8 m. One advantage of the Magnetic Field Calculator is that you can specify the data of your survey for updated values.
Entering Magnetic Declination in SonarWiz
The best place to enter the magnetic declination is in the sensor configuration before your survey, so that your raw sonar files have corrected headings. However, if you need to add them in post-processing, you can do so in SonarWiz. Select the sonar line and look for the Projection and smoothing section of the file properties. Enter the declination offset in the Port and Starboard rotation as shown in Figure 4:
